(1) Field of the Invention
The invention relates to the field of Chemical-Mechanical Polishing more particularly to ways to improve uniformity of material removal.
(2) Description of the Prior Art
In integrated circuit technology, the removal of various layers is usually accomplished through use of liquid or gaseous etchants. In these cases, the reaction products are fluids that are readily removed from the reaction site so that etching can proceed at a uniform rate. In certain cases, however, the reaction products are insoluble solids that are, at best, hard to remove. At worst, such etchants serve only to undermine, or weaken, the layer""s integrity near the surface and, in the absence of any other action, are thus quite ineffective as etchants.
In etching situations of this sort, mechanical assistance in the form of a slurry comprising an abrasive powder suspended in a suitable liquid medium (such as the etchant itself) can be used to complement the action of the chemical etchant. This technique for removing material through a combination of chemical and mechanical means is referred to as Chemical Mechanical Polishing (CMP).
While CMP makes possible the controlled removal of materials that could not be effectively removed in any other way, it tends to be slow unless relatively large amounts of slurry are used. There is thus relatively little time for slurry to be evenly distributed before it, together with any process byproducts, has to be removed and replaced with fresh slurry. It follows that for CMP to achieve uniform material removal rates over large areas, slurry must be evenly distributed as quickly as possible, once it has been dispensed onto the polishing surface.
In the prior art, rapid distribution of slurry has been achieved by simultaneously rotating both the platen (on which the slurry bearing pad sits) and the substrate (usually a silicon wafer) that is to be polished. An example of this is shown in FIG. 1 which has been taken from a patent by Burke et al. (U.S. Pat. No. 5,492,504 February, 1996). Wafer holder 2 keeps a silicon wafer (not seen) pressed up against pad 1. Said pad is supported by a platen (not seen) that is directly beneath it. Means for rotating the platen (not shown) cause it to move in the direction symbolised by arrow 9.
Still referring to FIG. 1, rotation causing means 3 (such as an in-situ motor), held in position by support arm 17, causes wafer holder 2 to rotate in the direction symbolised by arrow 19 while at the same time keeping the wafer to be polished pressed against pad 1. Slurry is dispensed onto pad 1 from slurry dispenser 4 while both pad and wafer are rotating. Slurry is thus swept along the outside edge of pad 1 to an outside edge of the wafer at which point the rotation of the wafer further distributes the slurry across the wafer.
A slightly different approach for achieving uniform slurry distribution is illustrated in FIG. 2 which has been taken from a patent by Regh et al. (U.S. Pat. No. 3,615,955 October, 1971). Pad 16, on platen 10, rotates as indicated by arrow 29. Wafer holder 20, holding multiple wafers, such as 25, is caused to rotate about center of rotation 28 in direction 99. Arm 22 serves to stabilize 20 while it rotates. Slurry is dispensed through opening 32 in a slurry supply bottle (not shown) that is suspended above the pad. Used slurry is removed at drain 12.
It is important to note that in the Regh system shown in FIG. 2 each wafer 25 rotates only once for each orbit about center of rotation 28. It has been our observation that a better distribution of slurry (resulting in a more uniform removal rate across the surface of a wafer) can be achieved if the wafer(s) are able to rotate more than once per orbit.
It is an object of the present invention to provide a CMP process and apparatus that distributes slurry more evenly than does the prior art, without the need to reduce the rate at which slurry is dispensed.
A further object of the present invention has been that said improved slurry distribution lead to an improvement in the uniformity of material removal.
These objects have been achieved by providing a polishing system in which the substrate that is to be polished (generally a silicon wafer) is given three independent, simultaneously applied, modes of motion. These are: rotation of the platen, rotation of the wafer, and orbital motion of the wafer. A machine for realizing this is described which includes a crank that is used to cause the wafer to revolve in a closed path on the surface of the polishing pad at the same time that it moves around the circumference of the pad. A method for using this machine to perform CMP is described.